The present invention relates to a developing apparatus which is provided on a copying machine of an electrophotographic system, a printer and on a facsimile machine and develops an electrostatic image, a developing method and an image forming apparatus, and in particular, to an improvement of a developing agent stirring and conveying device in the developing apparatus.
As a developing method used in an electrophotographic apparatus, there are known a regular developing method employed in an ordinary electrophotographic copying machine and a reversal developing method used in a digital printer and a digital electrophotographic copying machine. The reversal developing method is one wherein a laser and an LED are generally used as a light source, and a latent image formed on an image carrying member through charging and exposure is developed to be a toner image by toner charged to be in the same polarity as that of charges on the image carrying member. For example, when polarity of charges on the image carrying member is negative, polarity of toner is also negative, and development is made by the use of a voltage difference created by exposure to form a toner image on the image carrying member.
A transfer-accepting material is charged by a transfer unit employing corona discharge to be in polarity opposite to that of toner after development processing, and a toner image on the image carrying member is transferred onto the transfer-accepting material. After that, voltage of a transfer material is lowered by AC corona discharge or DC corona discharge to lower attraction between the transfer-accepting material and the image carrying member, and thereby the transfer-accepting material is exfoliated from the surface of the image carrying member to be ready for the following process.
In a conventional developing apparatus, a rotary developing agent carrying member is arranged in the vicinity of a rotary image carrying member. The developing agent carrying member is formed to be in the state of a hollow cylinder and is housed in a developing unit main body having an opening on its side facing the image carrying member. On the developing agent carrying member, there is impressed developing bias voltage in which AC voltage of 2700V and 8000 Hz is superposed on DC voltage, for example, of -600V. The developing agent carrying member has therein a fixed magnetic field generating means, and the outer circumferential surface of the developing agent carrying member carries two-component developing agent wherein toner particles and magnetic particles (carrier) are mixed.
The developing apparatus has therein a developing unit main body housing therein two-component developing agent which is composed of the toner and carrier, a developing agent carrying member representing a rotatable developing agent conveyance means, a developing agent supply member which supplies developing agent on the developing agent carrying member, a developing agent layer thickness regulating member which regulates a thickness of a developing agent layer on the developing agent carrying member, and a developing agent stirring and conveying member which stirs replenishing toner and developing agent and conveys developing agents to the developing agent supply member.
Toner replenished in the developing unit main body from a toner replenishing device through a toner replenishment inlet opened on the top of the developing unit main body is stirred by the rotating developing agent stirring and conveying member and mixed with developing agent contained in the developing unit main body to be of uniform toner concentration, and is supplied on the outer circumferential surface of the developing agent carrying member by the rotating developing agent supply member.
In recent years, there is a strong demand for colors even in these fields, and a color image forming method of an electrophotographic system and an apparatus employing that method are intensively studied. Among them, there is watched an image forming system (the so-called KNC system) wherein a series of steps to conduct uniform charging and imagewise exposure on an image carrying member are repeated to form superposed color images which are then transferred collectively onto a transfer body, because the mechanism of the image forming system is compact and images with high image quality can be obtained, and many technologies are studied.
Typical patents are disclosed in TOKKAISHO No. 60-76766 and others, and TOKKAISHO No. 60-95456 discloses a technology of an image forming method to make superposed color images by repeating twice or more the step to develop, on a non-contact basis, a latent image formed on the image carrying member, through vibrating electric field that is formed between a developing agent conveying carrier and an image carrying member, wherein an electrostatic latent image is made through dot exposure by a laser beam and dots each being for yellow, magenta and cyan are superposed, thus better image quality can be obtained.
In the KNC system mentioned above wherein a thin developing agent layer with a thickness of about 200-600 .mu.m needs to be formed on the developing agent carrying member, it is extremely important, for obtaining stable images, to carry developing agents to the surface of the developing agent carrying member by stirring and conveying the developing agents without deteriorating them.
In an image forming apparatus such as a copying machine of an electrophotographic system and a printer, two-component developing agents in a developing apparatus which are stirred insufficiently and deteriorated cause problems including insufficient charging of developing agents, a fall of image density, mixing of color (neglected mixing of color) of toner images which is caused when image forming is resumed after suspension of image forming for a long time, a fall of reproduction of fine lines, contamination inside and outside a developing apparatus caused by scattering of suspended toner, and toner image failure (character ruggedness) on fine lines and on edge sections.
It is necessary to make a developing apparatus small for the purpose of miniaturizing an image forming apparatus such as a copying machine of an electrophotographic system and a printer. In a color image forming apparatus equipped with plural developing apparatuses, in particular, it is necessary to miniaturize a developing apparatus also for attaining an image carrying member having a small diameter. When a developing apparatus is made small, a space for housing a developing agent stirring and conveying member which stirs, mixes and conveys toner replenished in the developing apparatus and developing agent in the developing apparatus, is made small. Therefore, the developing agent stirring and conveying member needs to be made small. However, in the case of a conventional miniaturized developing agent stirring and conveying member, insufficient stirring is caused, uniform toner density can not be obtained due to insufficient stirring and mixing of developing agents, and distribution of charging amount is broadened in the course of running, resulting in a fall of developability in the course of development processing, a fall of image density, neglected mixing of color, a fall of reproducibility of fine lines, character ruggedness, image fogging, and scattering of suspended toner, thus, images are deteriorated.